The present disclosure relates to a method of measuring the bonded perimeter width of a hollow component and particularly, but not exclusively, to a method of measuring the bonded perimeter width of a hollow fan blade for a gas turbine engine.
A turbofan gas turbine engine 10, as shown in FIG. 1, comprises in flow series an intake 11, a fan 12, an intermediate pressure compressor 13, a high pressure compressor 14, a combustion chamber 15, a high pressure turbine 16, an intermediate pressure turbine 17, a low pressure turbine 18 and an exhaust 19. The high pressure turbine 16 is arranged to drive the high pressure compressor 14 via a first shaft 26. The intermediate pressure turbine 17 is arranged to drive the intermediate pressure compressor 13 via a second shaft 28 and the low pressure turbine 18 is arranged to drive the fan 12 via a third shaft 30. In operation air flows into the intake 11 and is compressed by the fan 12. A first portion of the air flows through, and is compressed by, the intermediate pressure compressor 13 and the high pressure compressor 14 and is supplied to the combustion chamber 15. Fuel is injected into the combustion chamber 15 and is burnt in the air to produce hot exhaust gases which flow through, and drive, the high pressure turbine 16, the intermediate pressure turbine 17 and the low pressure turbine 18. The hot exhaust gases leaving the low pressure turbine 18 flow through the exhaust 19 to provide propulsive thrust. A second portion of the air bypasses the main engine to provide propulsive thrust.
The fan blades may be formed as hollow structural components comprising two metal sheet skins with a shaped core portion. The skins and core portion may be brazed together or may be diffusion bonded and superplastically formed.
In such blade constructions, the two blade skins are bonded to one another around a perimeter of the blade. This bonded region is sometimes colloquially referred to as a picture frame, and the width of the bonded region, in a direction normal to the edge, is sometimes colloquially referred to as the picture frame width. This is illustrated in FIG. 2 by the line B-C.
This picture frame width is an important parameter for the structural integrity of the formed fan blade because it provides a measure of the strength of the joint between the two blade skins.
The measurement of the picture frame width (the distance B-C in FIG. 2) is carried out manually using an ultrasonic probe. For each blade a series of measurement points are marked around the perimeter of the blade, and at each of the points a section line is drawn onto the blade surface in a direction normal to the blade edge. This is illustrated in FIG. 3.
At each of these section lines, the ultrasonic probe is traversed along the line from the edge of the blade towards the interior of the blade until the signal from the ultrasonic probe indicates that the interior edge of the cavity has been found. This point is marked on the blade surface and its distance from the edge is recorded.
This technique is time consuming, taking approximately 15 to 20 minutes for each blade.
The use of an existing non-contacting ultrasonic skin thickness measurement system has been proposed, as illustrated in FIG. 4. However, while this system can reliably detect the internal cavity edge, corresponding to point A of FIG. 4, it is unable to detect the external edge, corresponding to point D of FIG. 4.
This is because there is no ultrasonic signal in the region F-D due to the curved surface of the blade skin. Consequently, the blade skin thickness disappears at the point F and the distance F-D is not available.